1. Field of the Invention
The present invention generally relates to an optical module, and more particularly to an optical module for performing photoelectric conversion in a high-speed optical communication system.
2. Description of the Related Art
Recently, transmission rates required in optical communication systems are being increased with an increase in the amount of data to be transmitted. Additionally, it is required to reduce sizes and costs of apparatuses provided in the optical communication systems. Particularly, in high-speed optical communication systems, it is highly required to achieve miniaturization, high performance and lower costs of optical modules used for photoelectric conversion.
Many attempts have been made to reduce sizes and costs of devices used in low-speed communication systems by integrating circuits and using bare chips and discrete components together. Additionally, attempts have been started to reduce sizes and costs of devices used in medium-speed (2.4 Gbps, for example) communication systems. The techniques to achieve the miniaturization, high performance and lower costs of the devices have been applied to the optical modules.
FIG. 1 shows an example of a conventional optical module. In FIG. 1, a case 100 is made of metal and has a rectangular parallelepiped shape whose top surface is opened. A lid 110 is also made of metal and has a shape which can be engaged inside the case 100. An optical fiber cable 130 for inputting optical signals is connected to an optical component 120. The optical component 120 is fixed to the case 100 by a metal mount. Signals outputted from the optical component 120 are supplied to a printed-circuit board 140.
A high-speed circuit part 150 comprising a ceramic package is provided on the printed-circuit board 140. The printed-circuit board 140 processes low-speed signals and a part of high-speed signals. On the other hand, the high-speed circuit part 150 is dedicated to process the high-speed signals. The signals which are photoelectrically converted by the optical component 130 are so weak that the printed-circuit board 140 and the high-speed circuit part 150 are needed to be electrically shielded. Thus, the lid 110 is mounted inside the case 100 and sealed by seam welding so that the printed-circuit board 140 and the high-speed circuit part 150 are electrically shielded.
The seam welding is an electric resistance welding method used to connect metal members. Although the seam welding has an advantage that chips provided inside the package are not substantially heated, it has a drawback that the cost is high. However, characteristics of a surface acoustic wave (SAW) filter provided in the high-speed circuit part 150 are unstable and a required specification is not satisfied unless the SAW filter is used in a sealed structure. Additionally, although an IC containing a phase-locked loop (PLL) circuit may be substituted for the SAW device, such an IC has poor noise resistance and reliability as compared to the SAW filter. Thus, the seam welding is advantageously used to seal the case 100 and the lid 110, resulting in a high cost of the sealing.
Additionally, if all of the circuits are provided in a package to be sealed, a size of the package becomes large and thus the miniaturization and the cost reduction cannot be achieved.
Further, amplifiers used in an optical module which operates at a transmission rate of several Gbps are required to have a wide-range amplifying characteristic ranging from several tens of kHz to several GHz since an optical communication system generally uses a base band transmission. Thus, it is necessary to suppress impedance mismatch and generation of heat, which may be tolerated in a case of a low-speed transmission system, so that the wide-range amplifying characteristic is not degraded at a part connecting the optical component and the high-speed electric amplifier.
Still further, if a phase balance between the IC in the package to be sealed and the SAW filter is changed during the sealing process, fine adjustments of phase cannot be performed. Additionally, impedance mismatch occurs when a high-frequency transmission path passes through many layers of a multilayered board between the optical component and a main board.